1. Field of the Invention
The present invention relates to semiconductor devices, particularly a semiconductor device including a resistance storage element storing data according to level change of the resistance value. More particularly, the present invention relates to a semiconductor device including a phase change element storing data according to level change of the resistance voltage in association with phase change.
2. Description of the Background Art
In recent years, advancement is seen in the development of phase change memories as a non-volatile memory allowing increased scale of integration and high-speed operation. In a phase change memory, data is stored taking advantage of level change of the resistance value in association with phase change of the phase change element. Writing data in a phase change element is carried out by conducting a current flow to the phase change element to generate heat.
This writing operation includes a reset operation and a set operation. In a reset operation, the phase change element is converted to an amorphous state of high resistance by maintaining the phase change element at a relatively high temperature. In a set operation, the phase change element is converted to a polycrystalline state of low resistance by maintaining the phase change element at a relatively low temperature for a sufficient long period. Reading out data of a phase change element is carried out by conducting a current flow in a range that does not cause change in the state of the phase change element, and identifying the high or low level of the resistance value of the phase change element (for example, refer to 2002 IEEE International Solid-State Circuits Conference, Digest of Technical Papers, pp. 202-203).
The conventional phase change memory is disadvantageous in that the resistance value of the phase change element will vary even in the period during which a writing operation is not carried out.